Method and installation for producing carbon monoxide by cryogenic distillation

ABSTRACT

The invention relates to a method for producing carbon monoxide by cryogenic distillation. Said method comprises the following steps: a gaseous mixture ( 1 ) containing carbon monoxide, hydrogen and nitrogen is cooled and partially condensed to produce a cooled and partially condensed gaseous mixture; the cooled and partially condensed gaseous mixture ( 5 ) is separated in order to produce a gas enriched with hydrogen and a liquid enriched with carbon monoxide ( 11 ); a flow of the liquid enriched with carbon monoxide is sent to a product stripper ( 17 ) in order to produce hydrogen-depleted liquid carbon monoxide ( 19 ) and hydrogen-enriched gaseous carbon monoxide (RSD); a flow of the hydrogen-depleted carbon monoxide is sent to a first intermediate level of a distillation column ( 23 ); a liquid flow enriched with methane ( 27 ) compared to the flow supplied to the distillation column is drawn off from the bottom of the distillation column; a flow ( 29 ) enriched with carbon monoxide is drawn off at a second intermediate point above the first intermediate point; and a flow (RSDN2) enriched with nitrogen and optionally hydrogen compared to the flow supplied to the distillation column is drawn off from the top of the distillation column.

The present invention relates to a method and an installation forproducing carbon monoxide by cryogenic distillation.

The synthesis gas produced by partial oxidation or by reforming oftencontains a percentage of nitrogen.

All the percentages mentioned are mole percentages.

It is known to reduce the percentage of nitrogen in the carbon monoxideproduced by using a nitrogen and carbon monoxide separating column(EP-A-928936).

It is one object of the present invention to provide a cryogenicseparation installation for producing carbon monoxide substantiallycontaining no nitrogen using fewer columns than the installations of theprior art.

According to one object of the present invention, a method is providedfor producing carbon monoxide by cryogenic distillation comprising thefollowing steps:

i) a gas mixture containing carbon monoxide, hydrogen and nitrogen iscooled and partially condensed to produce a cooled and partiallycondensed gas mixture

ii) the cooled and partially condensed gas mixture is separated toproduce a hydrogen-enriched gas and a carbon monoxide-enriched liquid

iii) a stream of the carbon monoxide-enriched liquid is sent to astripping column to produce hydrogen-free liquid carbon monoxide andhydrogen-enriched carbon monoxide gas

iv) a stream of the hydrogen-free carbon monoxide is sent to a firstintermediate level of a distillation column

v) a liquid stream, richer in methane compared to the stream feeding tothe distillation column, is withdrawn from the bottom of thedistillation column

vi) a carbon monoxide-rich stream is withdrawn from a secondintermediate point, the second intermediate point being above the firstintermediate point

vii) a stream, enriched with nitrogen and optionally hydrogen comparedto the stream feeding to the distillation column, is withdrawn from thetop of the distillation column

Optionally:

-   -   the carbon monoxide-rich stream withdrawn from the distillation        column is a liquid stream;    -   a carbon monoxide cycle cools the top of the distillation column        and/or heats the bottom of the distillation column and/or heats        the bottom of the stripping column;    -   cycle carbon monoxide is expanded in a turbine.

According to a further object of the invention, an installation isprovided for producing carbon monoxide by cryogenic distillationcomprising:

a) a heat exchanger for cooling and partially condensing a gas mixturecontaining carbon monoxide, hydrogen and nitrogen to produce a cooledand partially condensed gas mixture

b) a separator for separating the cooled and partially condensed gasmixture to produce a hydrogen-enriched gas and a carbonmonoxide-enriched liquid

c) means for conveying the cooled and partially condensed gas mixturefrom the heat exchanger to the separator

d) a stripping column and means for conveying at least part of thecarbon monoxide-enriched liquid thereto

e) means for withdrawing a hydrogen-enriched gas from the top of thestripping column and means for withdrawing a hydrogen-free liquid fromthe bottom of the stripping column

f) a distillation column, means for sending a stream of thehydrogen-free liquid to a first intermediate point of the distillationcolumn, means for withdrawing a bottom liquid from the distillationcolumn, means for withdrawing an overhead gas from the distillationcolumn and means for withdrawing an intermediate fluid at a secondintermediate point of the distillation column, the second intermediatepoint being above the first intermediate point.

Optionally:

-   -   the distillation column has a top condenser and/or a bottom        reboiler;    -   the stripping column has a bottom reboiler;    -   the column (columns) is (are) heated and/or cooled using a        carbon monoxide gas cycle.

The invention is described in greater detail with reference to thesingle FIGURE.

In the FIGURE, a synthesis gas 1 containing 40% hydrogen, 58% carbonmonoxide, 0.1% methane, 0.5% nitrogen and 1.3% argon.

This gas is cooled in a heat exchanger 3 to form a cooled stream 5 thatis sent to a separator pot 7 where it is partially condensed. Thehydrogen rich gas formed 9 is heated in the heat exchanger 3 and sent toa consumer.

The liquid 11 is expanded in a valve 13 to form a stream 15. The liquidstream 15 is sent to the top of a stripping column 17 heated at thebottom by a stream Q1 sent to the bottom reboiler, which may, forexample, be part of a carbon monoxide cycle. A hydrogen-enriched wastegas stream RSD leaves the top of the stripping column 17. Ahydrogen-free liquid stream 19 is withdrawn from the bottom of thecolumn 17, expanded in the valve 21 and sent to an intermediate point ofa distillation column 23. The column 23 is heated at the bottom by astream Q2 sent to the bottom reboiler and cooled at the top by a streamQ3 sent to the top condenser, the two streams possibly forming part of acarbon monoxide cycle.

A methane-rich liquid stream 27 is withdrawn from the bottom of thecolumn 23. A carbon monoxide-rich liquid stream 29 is withdrawn at anintermediate point above the point of arrival 19 of the liquid. Thisliquid preferably contains 98.5% carbon monoxide, less than 1% argon,less than 10⁻² ppm methane and 0.4% nitrogen. A gas mixture of hydrogenand nitrogen RSD N2 is withdrawn from the top of the distillation column3.

The stream RSD N2 is mixed with the stream RSD from the column 17 andleaves the installation to be discharged to the atmosphere and/or burnedafter being heated in the heat exchanger 3.

The liquid 29 joins a stream of liquid 31 issuing from the cycle and themixed stream is expanded in a valve 33 before being heated in the heatexchanger 3 to form the product 35.

The installation is kept cold by a carbon monoxide gas turbine 37,expansion of another fluid and/or by liquid injection.

1-8. (canceled)
 9. A method for producing carbon monoxide by cryogenicdistillation comprising the following steps: a) a gas mixture containingcarbon monoxide, hydrogen and nitrogen is cooled and partially condensedto produce a cooled and partially condensed gas mixture; b) the cooledand partially condensed gas mixture is separated to produce ahydrogen-enriched gas and a carbon monoxide-enriched liquid; c) a streamof the carbon monoxide-enriched liquid is sent to a stripping column toproduce hydrogen-free liquid carbon monoxide and hydrogen-enrichedcarbon monoxide gas; d) a stream of the hydrogen-free carbon monoxide issent to a first intermediate level of a distillation column; e) a liquidstream, richer in methane compared to the stream feeding to thedistillation column, is withdrawn from the bottom of the distillationcolumn; f) a carbon monoxide-rich stream is withdrawn from a secondintermediate point, the second intermediate point being above the firstintermediate point; and g) a stream, enriched with nitrogen andoptionally hydrogen compared to the stream feeding to the distillationcolumn, is withdrawn from the top of the distillation column.
 10. Themethod of claim 9, in which the carbon monoxide-rich stream withdrawnfrom the distillation column is a liquid stream.
 11. The method of claim9, in which a carbon monoxide cycle cools the top of the distillationcolumn and/or heats the bottom of the distillation column and/or heatsthe bottom of the stripping column.
 12. The method of claim 9, in whichcycle carbon monoxide is expanded in a turbine.
 13. An installation forproducing carbon monoxide by cryogenic distillation comprising: a) aheat exchanger for cooling and partially condensing a gas mixturecontaining carbon monoxide, hydrogen and nitrogen to produce a cooledand partially condensed gas mixture; b) a separator for separating thecooled and partially condensed gas mixture to produce ahydrogen-enriched gas and a carbon monoxide-enriched liquid; c) meansfor conveying the cooled and partially condensed gas mixture from theheat exchanger to the separator; d) a stripping column and means forconveying at least part of the carbon monoxide-enriched liquid thereto;e) means for withdrawing a hydrogen-enriched gas from the top of thestripping column and means for withdrawing a hydrogen-free liquid fromthe bottom of the stripping column; and f) a distillation column, meansfor sending a stream of the hydrogen-free liquid to a first intermediatepoint of the distillation column, means for withdrawing a bottom liquidfrom the distillation column, means for withdrawing an overhead gas (RSDN2) from the distillation column and means for withdrawing anintermediate fluid at a second intermediate point of the distillationcolumn, the second intermediate point being above the first intermediatepoint.
 14. The installation of claim 13, in which the distillationcolumn has a top condenser and/or a bottom reboiler.
 15. Theinstallation of claim 13, in which the stripping column has a bottomreboiler.
 16. The installation of claim 13, in which the column(columns) is (are) heated and/or cooled using a carbon monoxide gascycle.